1. Field of the Invention
This invention relates to an X-ray radiographic apparatus providing a solid-state image sensing device for photographing an X-ray radiographic image.
2. Description of Related Art
Generally, an automatic X-ray exposure unit is provided in an X-ray radiographic apparatus to obtain an adequate density of the X-ray picture. In this automatic X-ray exposure unit, X-rays transmitted through an object is detected by detecting means, and if the detected total quantity of X-rays reaches a predetermined value, irradiation by X-rays is stopped. In this manner, adequate density of the X-ray picture is obtained. One type of automatic X-ray exposure unit uses an Image Intensifier (I.I.) as a detecting means. It includes a phototimer in the rear of the I.I. to control X-ray exposure time in accordance with a thickness of the object or a portion thereof.
In a conventional X-ray radiographic apparatus, an image pick up tube has been used to pick up image. However, an image pick up tube has significant limitations relating to pixel number and reading time. For this reason, solid-state image sensing devices, such as a charge-coupled device (CCD) have come into used as a substitute for the image pick up tube.
In general, one frame period is fixed to a predetermined period of time, such as 1/30 sec., as in a TV camera. X-ray exposure time is varied in accordance with the thickness of the object or a portion thereof. Occasionally X-ray exposure time exceeds one frame period (1/30 sec.). Accordingly, if the X-ray exposure time is controlled automatically, sometimes the charge is read out before the detected total quantity of X-rays reaches a desired level.
In this type of X-ray radiographic apparatus, it is necessary to add plural frames to obtain adequate density of an X-ray image. The use of plural frames makes the X-ray radiographic apparatus complex.